Hot press-formed product and process for producing same

Abstract

There is provided a hot press-formed product, including a thin steel sheet formed by a hot press-forming method, and having a metallic structure that contains bainitic ferrite at 70% to 97% by area, martensite at 27% by area or lower, and retained austenite at 3% to 20% by area, the remainder structure of which is at 5% by area or lower, whereby balance between strength and elongation can be controlled in a proper range and high ductility can be achieved.

The invention claimed is: 
     
       1. A hot press-formed product, comprising a thin steel sheet formed by a hot press-forming method, and having a metallic structure that contains bainitic ferrite at 70% to 97% by area, as-quenched martensite at 5% to 27% by area, and retained austenite at 3% to 20% by area, the remainder structure of which is at 5% by area or lower, having the following chemical element composition:
 C at 0.15% to 0.4% (where “%” means “% by mass”, and the same applies to the below with respect to the chemical element composition); 
 Si at 0.5% to 3%; 
 Mn at 0.5% to 2%; 
 P at 0.05% or lower (not including 0%); 
 S at 0.05% or lower (not including 0%); 
 Al at 0.01% to 0.1%; 
 Cr at 0.01% to 1%; 
 B at 0.0002% to 0.01%; 
 Ti at (N content)×4% to 0.1%; and 
 N at 0.001% to 0.01%, 
 and the remainder consisting of iron and unavoidable impurities. 
 
     
     
       2. The hot press-formed product according to  claim 1 , further comprising, as additional elements, one or more selected from the group consisting of Cu, Ni, and Mo at 1% or lower (not including 0%) in total. 
     
     
       3. The hot press-formed product according to  claim 1 , further comprising, as additional elements, V and/or Nb at 0.1% or lower (not including 0%) in total. 
     
     
       4. A process for producing a hot press-formed product as set forth in  claim 1 , comprising:
 heating a thin steel sheet to a temperature not lower than Ac 3  transformation point and not higher than 1000° C.; and then 
 starting the forming of the thin steel sheet with a press tool to produce the hot press-formed product, during which forming an average cooling rate of 20° C./sec or higher is kept in the press tool and the thin steel sheet is cooled to a temperature range of not lower than martensite transformation starting temperature (Ms), and which forming is finished after retention in the temperature range for 10 seconds or longer.